fault plane measured against the vertical is called the _hade_ of the fault. The _total displacement_ is measured along the plane of the fault from a point upon one limb to the point from which it was separated in the other. The additional terms are made sufficiently clear by the diagram. [Illustration: FIG. 42.—A fault in previously disturbed strata. _AB_, displacement; _AC_, throw; _BD_, stratigraphic throw; _BC_, heave; angle _CAB_, hade.] =Methods of detecting faults.=—The first effect of a fault is usually to produce a crack at the surface of the earth; and, provided there is a vertical displacement or throw, an escarpment which rises upon the upthrown side of the fault. In general it may be said that escarpments which appear at the earth’s surface as plane surfaces probably represent planes of fracture, though not necessarily planes of faulting. In many cases the actual displacements lie buried under loose rock débris near to and paralleling the escarpment, and in some cases as a result of the erosional processes working upon alternately hard and soft layers of rock, the escarpment may later appear upon the downthrown side or limb of the fault (Fig. 43). As an illustration of a fault escarpment, the façade of El Capitan and many other rock faces of the Yosemite valley may be instanced. [Illustration: FIG. 43.—Diagrams to show how an escarpment originally on the upthrown side of the fault may, through erosion, appear upon the downthrown side.] [Illustration: FIG. 44.—A fault plane exhibiting “drag.” The opening is artificial (after Scott).] When we have further studied the erosional processes at the earth’s surface, it will be appreciated that faults tend to quickly bury themselves from sight, whereas fold structures will long remain in evidence. Many faults will thus be overlooked, and too great weight is likely to be ascribed to the folds in accounting for the existing attitudes and positions of the rock masses. Faults must therefore be sought out if mistakes of interpretation are to be avoided.